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AP Biology 2007-2008

Chapter 12: The Cell Cycle:

(2)

AP Biology

Function of Cell Division

making new cells

growth

repair & renew

continuity of life

asexual reproduction

amoeba

(3)

AP Biology 2005-2006

Cell division / Asexual reproduction

Mitosis

produce cells with same information

identical daughter cells

exact copies

clones

same amount of DNA

same number of chromosomes

same genetic information

Aaaargh!

I’m seeing double!

(4)

2005-2006

Asexual reproduction

All prokaryotes reproduce asexually

Single celled eukaryotes

– yeast

– Paramecium – Amoeba

Simple multicellular eukaryotes reproduce asexually

– Hydra

• budding

What are the disadvantages of asexual reproduction?

What are the advantages?

(5)

AP Biology 2005-2006

Budding in Yeast

Binary fission in Amoeba

(6)

AP Biology

nuclear pores

nuclear pore

nuclear envelope nucleolus

histone protein

chromosome DNA

Function

protects DNA

Structure

nuclear envelope

double membrane

membrane fused in spots to create pores

allows large macromolecules to pass through

Nucleus

What kind of molecules need to

pass through?

(7)

Cytoskeleton

microfilaments, intermediate filaments, microtubules microtubules

Chromosomes movement in cell division

microfilament

Cell division, cleavage furrow formation

(8)
(9)

Centrioles

• Found in animal cells only

• organize microtubules and form spindle fiber.

Spindle itself is made of microtubules

• guide chromosomes in mitosis

Cross section of a centriole 9+2 arrangement

(10)

AP Biology

Getting the right stuff

What is passed on to daughter cells?

exact copy of genetic material = DNA

mitosis

division of organelles & cytoplasm

cytokinesis

chromosomes (stained orange) in kangaroo rat epithelial cell

notice cytoskeleton fibers

(11)

includes

is divided into is divided into

Concept Map

Section 10-2

Cell Cycle

M phase (Mitosis) Interphase

G1 phase S phase G2 phase Prophase Metaphase Anaphase Telophase

Cytokinesis

(12)

End Show

10-2 Cell Division

Slide 12 of 38

Copyright Pearson Prentice Hall

Cell Cycle

Events of the Cell Cycle

(13)

AP Biology

Overview of mitosis

interphase prophase (pro-metaphase)

metaphase anaphase telophase

cytokinesis

I.P.M.A.T.

(14)

DNA packing

The double helix of DNA is highly negatively charged due to all the negatively charged

phosphates in the backbone.

Histones are positively charged proteins that

wrap up DNA through interactions between their positive charges and negative charges of DNA.

Double-stranded DNA loops around 8 histones twice,

forming the nucleosome,

which is the building block of chromatin packaging.

(15)

DNA Packing

DNA is packaged by forming coils of nucleosomes, called chromatin fibers. These fibers are condensed

into chromosomes during mitosis, or the process of cell division.

(16)

AP Biology

Copying DNA & packaging it…

After DNA duplication, chromatin condenses

coiling & folding to make a smaller package

DNA

chromatin

mitotic chromosome

(17)

AP Biology

double-stranded mitotic human chromosomes

(18)

AP Biology

Mitotic Chromosome

Duplicated chromosome

2 sister chromatids

narrow at centromeres

contain identical

copies of original DNA

single-stranded

double-stranded

(19)
(20)

Kinetochores are protein complexes associated with centromeres

In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule

organizing center

An aster (a radial array of short microtubules) extends from each centrosome

The spindle includes the centrosomes, the microtubules, and the asters

© 2011 Pearson Education, Inc.

(21)

Interphase

90% of cell life cycle

cell doing its “everyday job”

Divided into 3 phases:

– G1 = 1st Gap

• cell grows

– S = DNA Synthesis

– G2 = 2nd Gap

Prepare for mitosis

G0

(22)

Interphase

Nucleus well-defined

– DNA loosely packed in long chromatin fibers

(23)

Copyright Pearson Prentice Hall

G1 Phase

•the cell

– increases in size

– synthesizes new proteins and organelles

(24)

Copyright Pearson Prentice Hall

S Phase

• chromosomes are replicated by DNA synthesis

• The centrosome replicates forming two centrosomes that migrate to opposite ends of the cell during

prophase and prometaphase

–Once a cell enters the S phase, it usually completes the rest of the cell cycle.

(25)

End Show

10-2 Cell Division

Slide 25 of 38

Events of the Cell Cycle

The G2 Phase (Second Gap Phase)

Organelles, proteins, molecules, and membrane required for cell division

Once G2 is complete, the cell is ready to start the M phase—Mitosis

(26)

AP Biology

Mitosis

Dividing cell’s DNA between 2 daughter nuclei

“dance of the chromosomes”

4 phases

prophase

metaphase

anaphase

telophase

(27)

AP Biology

Prophase

Chromatin condenses

visible chromosomes

chromatids

Centrioles move to opposite poles of cell

Protein fibers cross cell to form mitotic spindle

microtubules coordinates movement of chromosomes

Nucleolus disappears

Nuclear membrane breaks down

(28)

AP Biology

Transition to Metaphase

Prometaphase

spindle fibers (microtubules) attach to centromeres at

kinetochores

chromosomes begin moving

(29)

Metaphase

Chromosomes align along middle of cell

– metaphase plate - the imaginary midway point between two spindle poles.

• meta = middle

Centrioles migrate to

opposite poles in the cell, riding along on the

developing spindle.

Nonkinetochore

microtubules (or polar Micortubules) from

opposite poles overlap each other extensively during metaphase.

(30)

AP Biology

spindle fibers coordinate movement and helps to ensure chromosomes separate properly so each new nucleus receives only 1 copy of each chromosome

(31)

Anaphase

Separation of chromatids

In anaphase, proteins holding together sister chromatids are inactivated

– Sister chromatids separate at kinetochores to become individual chromosomes

2 chromosomes 1 chromosome

2 chromatids

single-stranded double-stranded

(32)

AP Biology

Anaphase

Sister chromatids move to opposite poles

pulled at centromeres

pulled apart by motor proteins interacting with kinetochore microtubules

(33)

Centromeres move toward the poles as the

microtubules that connect them shorten. This

shortening is due to the

removal of tubulin subunits from the kinetochore ends (+ end) of the microtubules.

Chromosome movement

(34)

• During anaphase region of overlap is reduced as motor proteins attached to the microtubules walk them away from one another using ATP.

• As microtubules push themselves apart from each other, their spindle poles are pushed apart, elongating the cell.

• At the same time, the microtubules continue to overlap by the addition of tubulin subunits to their overlapping ends.

Cell Elongation by Nonkinetochore (Polar) Microtubules

(35)

AP Biology

Telophase

Chromosomes arrive at opposite poles

daughter nuclei form

nucleoli form

chromosomes disperse

no longer visible under light microscope

Spindle fibers disperse

Cytokinesis begins

(36)

AP Biology

Cytokinesis in animals

cleavage furrow forms by constriction belt of actin microfilaments around equator of cell

splits cell in two

like tightening a draw string

(37)

AP Biology

Cytokinesis in Animals

(play Cells Alive movies here) (play Thinkwell movies here)

(38)

AP Biology

Mitosis in whitefish blastula

(39)

AP Biology

Mitosis in animal cells

(40)

Cytokinesis in Plants

Cell plate forms

vesicles derived from Golgi line up at equator

vesicles fuse to form 2 cell membranes

new cell wall laid down between membranes

– new cell wall fuses with existing cell wall

(41)

AP Biology

Cytokinesis in plant cell

(42)

AP Biology

Mitosis in plant cell

(43)

AP Biology

onion root tip

(44)

AP Biology

Overview of mitosis

interphase prophase (pro-metaphase)

metaphase anaphase telophase

cytokinesis

I.P.M.A.T.

(45)

AP Biology

G2

S G1

M metaphase prophase

anaphase telophase

interphase (G1, S, G2 phases) mitosis (M)

cytokinesis (C) C

Frequency of cell division varies by cell type

embryo

cell cycle < 20 minute

skin cells

divide frequently throughout life

12-24 hours cycle

liver cells

retain ability to divide, but keep it in reserve

divide once every year or two

mature nerve cells & muscle cells

do not divide at all after maturity

permanently in G0

Frequency of cell division

(46)

AP Biology

Overview of Cell Cycle Control

Two irreversible points in cell cycle

replication of genetic material

separation of sister chromatids

Checkpoints

process is assessed & possibly halted

centromere

sister chromatids

single-stranded

chromosomes double-stranded chromosomes

There’s no turning back,

now!

(47)

AP Biology

Checkpoint control system

3 major checkpoints:

G1 Phase checkpoint

can DNA synthesis begin?

G2 Phase checkpoint

has DNA synthesis been completed correctly?

commitment to mitosis

M phase checkpoint

are all chromosomes attached to spindle?

can sister chromatids separate correctly?

(48)

For many cells, the G1 checkpoint seems to be the most important

If a cell receives a go-ahead signal at the G1

checkpoint, it will usually complete the S, G2, and M phases and divide

If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing

state called the G0 phase

© 2011 Pearson Education, Inc.

(49)

AP Biology

Cell cycle

M Mitosis

G1 Gap 1

G0 Resting G2

Gap 2

S Synthesis

Cell has a “life cycle”

cell is formed from a mitotic division

cell grows & matures to divide again

cell grows & matures to never divide again

G1, S, G2, M G1G0

epithelial cells, blood cells,

stem cells

brain / nerve cells muscle cells liver cells

(50)

AP Biology

G0 phase

M Mitosis

G1 Gap 1

G0 Resting G2

Gap 2

S Synthesis

G0 phase

non-dividing, differentiated state

most human cells in G0 phase

liver cells

in G0, but can be

“called back” to cell

cycle by external cues

nerve & muscle cells

highly specialized

arrested in G0 & can never divide

(51)

The Cell Cycle Clock: Cyclins and Cyclin- Dependent Kinases

Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

Cdks activity fluctuates during the cell cycle because it is controled by cyclins, so named because their concentrations vary with the cell cycle

MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

© 2011 Pearson Education, Inc.

(52)

AP Biology

Cell cycle signals

Cell cycle controls

cyclins

regulatory proteins

levels cycle in the cell

Cdk’s

cyclin-dependent kinases

phosphorylates cellular proteins

activates or inactivates proteins

Cdk-cyclin complex

triggers passage through different stages of cell cycle

(53)

Figure 12.17

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

(b) Molecular mechanisms that help regulate the cell cycle MPF activity

Cyclin

concentration

Time

M G1 S G2 M G1 S G2 M G1

Cdk Degraded

cyclin

Cyclin is degraded

MPF G2

checkpoint Cdk

Cyclin

(54)

AP Biology

During Anaphase, MPF switches itself off by starting a process that leads to destruction of cyclin molecules.

Without cyclin molecules Cdk

molecules become inactive, bringing mitosis to a close.

How does cell stop cell division?

(55)

AP Biology

Cyclin & Cyclin-dependent kinases

CDKs & cyclin drive cell from one phase to next in cell cycle

proper regulation of cell cycle is so key to life that the genes for these regulatory proteins

have been highly conserved through evolution

the genes are basically the same in yeast,

insects, plants &

animals (including humans)

(56)

AP Biology

External signals

Growth factors

coordination between cells

protein signals released by

body cells that stimulate other cells to divide

density-dependent inhibition

crowded cells stop dividing

each cell binds a bit of growth factor

not enough activator left to trigger division in any one cell

anchorage dependence

to divide cells must be attached to a substrate

“touch sensor” receptors

(57)

AP Biology

E2F

nucleus

cytoplasm

cell division

nuclear membrane

growth factor

protein kinase cascade

nuclear pore

chromosome

cell surface Cdk

receptor

P

P P

P P

Growth factor signals

(58)

AP Biology

Example of a Growth Factor

Platelet Derived Growth Factor (PDGF)

made by platelets in blood clots

binding of PDGF to cell receptors stimulates cell division in fibroblast (connective tissue)

heal wounds

Don’t forget to mention erythropoietin!

(EPO)

(59)

AP Biology

Growth Factors and Cancer

Growth factors can create cancers

proto-oncogenes

normal growth factor genes that become oncogenes (cancer-causing) when mutated

stimulates cell growth

if oncogenes are turned “ON” can cause cancer

example: RAS (activates cyclins)

tumor-suppressor genes

inhibits cell division

if switched “OFF” can cause cancer

example: p53

(60)

AP Biology

DNA damage is caused by heat, radiation, or chemicals.

p53 allows cells with repaired DNA to divide.

Step 1

DNA damage is caused by heat, radiation, or chemicals.

Step 1 Step 2

Damaged cells continue to divide.

If other damage accumulates, the cell can turn cancerous.

Step 3

p53 triggers the destruction of cells damaged beyond repair.

ABNORMAL p53 NORMAL p53

abnormal p53 protein

cancer cell

Step 3

The p53 protein fails to stop cell division and repair DNA.

Cell divides without repair to damaged DNA.

Cell division stops, and p53 triggers enzymes to repair damaged region.

Step 2

DNA repair enzyme p53

protein

p53 protein

p53 — master regulator gene

(61)

AP Biology

Development of Cancer

Cancer develops only after a cell experiences

~6 key mutations (“hits”)

unlimited growth

turn on growth promoter genes

ignore checkpoints

turn off tumor suppressor genes (p53)

escape apoptosis

turn off suicide genes

immortality = unlimited divisions

turn on chromosome maintenance genes

promotes blood vessel growth

turn on blood vessel growth genes

overcome anchor & density dependence

turn off touch-sensor gene

It’s like an out of control

car!

(62)

AP Biology

What causes these “hits”?

Mutations in cells can be triggered by

UV radiation

chemical exposure

radiation exposure

heat

cigarette smoke

pollution

age

genetics

(63)

AP Biology

Tumors

Mass of abnormal cells

Benign tumor

abnormal cells remain at original site as a lump

p53 has halted cell divisions

most do not cause serious problems &

can be removed by surgery

Malignant tumors

cells leave original site

lose attachment to nearby cells

carried by blood & lymph system to other tissues and start more tumors = metastasis

impair functions of organs throughout body

(64)

The Evolution of Mitosis

Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission

Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

© 2011 Pearson Education, Inc.

(65)

Figure 12.13

(a) Bacteria

(b) Dinoflagellates

(d) Most eukaryotes

Intact nuclear envelope

Chromosomes Microtubules

Intact nuclear envelope

Kinetochore microtubule

Kinetochore microtubule

Fragments of

nuclear envelope Bacterial

chromosome

(c) Diatoms and some yeasts

References

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